An electric dipole is held in a uniform electric field.

a) Show that the net force acting on it is zero.

b) The dipole is aligned parallel to the field.Find the work done in rotating through angle of 180 degree

Please refer to the link given below to get the answer for part (a):-
https://www.meritnation.com/ask-answer/question/an-electric-dipole-is-held-in-a-uniform-electric-field/electric-charges-and-fields/3329007
(b) When dipole is aligned parallel to the electric field. i.e., θ₁= 0⁰ and suppose we have to set it an angle θ with $\underset{E}{\to }$ i.e., θ₂ = 180⁰.
As the potential energy  for a dipole in a uniform electric field is given by :-
P.E.= W= -pE (cos θ₂ - θ₁) = -pE(cos180⁰ - cos 0⁰ ) = 2pE